The invention relates to pulverizing systems for grinding various materials into smaller particles. It finds particular application in conjunction with an improved disc mill assembly in the pulverizing system and will be described with particular reference thereto. Typically such pulverizing systems are used to grind pelletized or shredded plastics, nylons, polyesters, and other polymers into respective powders. However, it is to be appreciated that the invention is also amenable to other applications.
Pulverizing systems with one or more disc mill assemblies are well known. Generally, such systems include a means for feeding input material to the disc mill, a means for carrying ground material from the disc mill to a sorting module, a means for transporting acceptable ground material to a ground material collection area, and a means for recirculating unacceptable ground material to a disc mill for further grinding. A current disc mill typically includes a spindle, a flywheel, a rotating disc blade, a stationary disc blade, a means for cooling the stationary disc blade (e.g., a waterjacket), a means for introducing air into the mill, and a means for adjusting a gap between facing cutting surfaces of the disc blades.
Previously, the blades in disc mills were heavy in order to withstand various pressures and forces exerted on the blade. The blades were also previously held in place with bolts securing the rotating blade to the flywheel and the stationary blade to another fixed component integrated with the water jacket used to cool the blade. A problem is that the bolt holes created a weak point in the blades thus conventional designs use a thicker and heavier blade to compensate for this weakness.
Using a stationary disc blade as an integral part of the water jacket creates several additional problems. The cavity in the water jacket for circulating coolant (e.g., water) was actually formed by the back surface of the blade and a mating surface on the other component defining the cavity. Seal members such as O-rings between the cavity component and the disc blade formed a seal for the coolant. Here, one problem is the O-rings must be replaced each time the disc blade is removed from the cavity component. Also, in each blade sharpening cycle, the back or rear surface of the stationary disc blade must be resurfaced to remove pitting caused by direct contact with the coolant. These issues add further downtime and expense to refurbishing the system.
Another inherent problem with the heavy disc blades are the operating costs associated with the blades. When material throughput for the pulverizing system becomes reduced to an unacceptable level, the disc blades must be replaced with new or re-sharpened blades. For some materials, for example, the disc blades become degraded after approximately 150 operating hours. A given disc blade, for example, is shipped to a supplier or third party for sharpening ten or more times before a minimum disposal thickness (e.g., ⅝ inch (1.58 cm)) is reached. The costs associated with continually sharpening the disc blades also include significant shipping costs. In some locations it costs more for shipping the disc blades back and forth than the cost of sharpening.
In prior mill assemblies, adjustment of the gap between the disc blades was controlled by a combination of adjustable spacers and attaching hardware. The spacers and attaching hardware, with respect to the lid and water jacket, were spaced apart from each other creating undesirable stresses on the water jacket and making it difficult to simultaneously adjust a given spacer and corresponding attaching hardware. Usually, an operator must repetitively loosen the attaching hardware and readjust the spacer before a desired gap is obtained due to the spaced apart configuration. Again, significant downtime of the system was associated with this trial-and-error type of adjustment.
Each material submitted to the pulverizing system for grinding has a temperature at which it is best ground. A preferred grinding temperature is normally slightly under the melting point of the material. If the grinding process temperature gets too high, the material being ground will liquify and plug up the mill housing causing a “melt down” which can damage the tooling inside the mill housing and the mill housing itself if not shut down immediately. This operating temperature inside the mill housing is very important.
A “maximum temperature allowable” (e.g., a predetermined temperature less the “melt down” temperature) is set at a control panel and, based on a monitored temperature of the material in the disc mill assembly, a motor controller controls the speed of a feeder. For example, if the material temperature is below the maximum temperature, the speed of the feeder may be increased until a motor associated with the disc mill assembly reaches a maximum amperage. When the material temperature reaches the maximum temperature, the controller cuts back on the speed of the feeder to reduce the material temperature. At that point, the temperature in the mill housing is controlling the throughput of the system, rather than the amperage draw of the motors. Currently, local ambient air flows into the mill housing through static air inlet holes in the housing, a material inlet to the housing, and various other points (i.e., the housing is not required to be air tight). Neither of these input air streams are adjustable. Moreover, neither of these input air streams provide air that is cooler than local ambient air.
Thus, there is a particular need for an improved disc mill assembly in a pulverizing system and for a new type of disc blades for an improved disc mill assembly. Moreover, the disc mill assembly requires improved temperature control/regulation to overcome the above-noted problems.